'''
You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

'''
# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        preNode = ListNode(0)
        currNode = preNode
        carry = 0
        while l1 != None or l2 != None:
            x = 0 if l1 == None else l1.val
            y = 0 if l2 == None else l2.val
            sum = x + y + carry
            # carry of the bit
            carry = sum / 10
            # sum: save into ListNode actually
            sum %= 10
            # save value into ListNode
            currNode.next = ListNode(sum)
            # move to the next
            currNode = currNode.next

            # move l1, l2 to the next
            if l1 != None:
                l1 = l1.next
            if l2 != None:
                l2 = l2.next
        
        # final carry check
        if carry == 1:
            currNode.next = ListNode(carry)
        
        return preNode.next